Short answer
HubSpot's KB: update connected domains when the website URL changes, update brand identity for pages, emails and sales assets, and update website settings so tracking, forms and CTAs keep working. AI brand voice needs Professional or Enterprise.
1. In HubSpot
The relevant screen in my demo portal.

2. Worth knowing
Check email sending domains too if the email domain changes.
3. Related
Brand kit and replacing a brand domain.